1. Field of the Invention
The present invention relates to light diffusion sheets which have directional light diffusion function to allow transmitted rays of light to diffuse while collecting to a side in a normal direction and which are particularly suitable for a backlight of a liquid crystal display device, and backlight units in which the light diffusion sheet is used.
2. Description of the Related Art
Liquid crystal display devices in widespread use have included a backlight system in which light emission is executed by irradiating onto a liquid crystal layer from the back face. In such a type of display devices, a backlight unit which is an edge light type, an immediate beneath type or the like is provided to an under face side of the liquid crystal layer. Such a backlight unit 50 of an edge light type is equipped with a rod-shaped lamp 51 for use as a light source, an optical waveguide plate 52 having a square plate shape disposed so that the edge thereof abuts along the lamp 51, a light diffusion sheet 53 disposed to the front face side of the optical waveguide plate 52, and a prism sheet 54 disposed to the front face side of the light diffusion sheet 53, as fundamentally shown in FIG. 4A.
Referring to functions of this backlight unit 50, rays of incident light from the lamp 51 to the optical waveguide plate 52 are first reflected on reflection dots or a reflection sheet (not shown in the Figure) of the back face of the waveguide plate 52, and exit from the front face of the waveguide plate 52. The rays of light exited from the waveguide plate 52 enter into the light diffusion sheet 53, then are diffused by the light diffusion sheet 53 and exit from the front face of the light diffusion sheet 53. Thereafter, the rays of light exited from the light diffusion sheet 53 enter into the prism sheet 54, and exit as rays of light having a distribution representing a peak in a direction along a substantially normal line via a prism part 54a formed on the front face of the prism sheet 54.
Accordingly, the rays of light exited from the lamp 51 are diffused by the light diffusion sheet 53, and refracted by the prism sheet 54 so that they represent a peak in a direction along the substantially normal line, and illuminate the entire face of the liquid crystal layer on the front face side (not shown in the Figure). Meanwhile, although not shown in the Figure, a light diffusion sheet may be further disposed to the front face side of the prism sheet 54 for the purpose of: relaxation of light condensing properties of the prism sheet 54 as described above; protection of the prism part 54a; or prevention of the sticking between the prism sheet 54 and the liquid crystal panel such as a polarization plate.
The light diffusion sheet 53 to be disposed to the backlight unit 50 generally has a transparent substrate layer 56 made of a synthetic resin, and a light diffusion layer 57 overlaid on the front face of the substrate layer 56 as shown in FIG. 4B (for example, see, JP-A Nos. H07-5305 and 2000-89007). In general, this light diffusion layer 57 includes resin beads 59 in a transparent resin binder 58, and the beads 59 exert the light diffusion function.
In recent years, characteristics desired for LCD vary depending on their applications, but may include (a) heightening of luminance, (b) widening of viewing angle, (c) energy saving, (d) thin and light modeling, (e) price reduction (economical efficiency) capabilities and the like. Particularly, at present, not only optical functions such as heightening of luminance, widening of viewing angle and the like, but also social needs such as price reduction, thin and light modeling capability, and the like have been increasing.
On the other hand, according to the conventional light diffusion sheet 53, means for increasing the amount of incorporated beads 59 and amount of the overlaid light diffusion layer 57 are generally believed to be appropriate as means for improving light diffusibility, however, such increase in amount of incorporating the beads 59 and amount of the overlaid light diffusion layer 57 may lead to difficulties in coating and decrease in transmittance of rays of light. Hence, taking into account of balance among light diffusibility, transmittivity of rays of light and coating facility, the conventional general light diffusion sheet 53 employs as the beads 59, polydisperse beads that have comparatively large mean particle size of approximately 20 μm, and have comparatively broad distribution of the particle size while having the weight ratio of the beads 59 to the binder of approximately 1 and the amount of the overlaid light diffusion layer 57 of 15 to 20 g/m2.
However, the aforementioned conventional light diffusion sheet 53 is formed such that hemispherical protruding parts are comparatively adjacent to the front face of the light diffusion layer 57 owing to the particle size distribution, mean particle size and amount of the beads 59. Therefore, continuous scratches may be generated on the front face of the light diffusion layer 57 in steps of producing the light diffusion sheet 53 and assembly of LCD, whereby reduction of process yield of the product may be caused. Such reduction of process yield of the product of the light diffusion sheet 53 shall be contrary to needs for price reduction of the LCD.